Nerve Impulse Conduction and Neuron Physiology

Nodes of Ranvier

  • Definition: Gaps between the segments of the myelin sheath on the axon.

  • Importance: Critical for nerve impulse conduction.

Myelin Sheath

  • Description: Acts like insulating beads along the axon, composed primarily of fat (lipids).

  • Appearance: White due to high fat content, similar to fat in meat (chicken/beef).

  • Function:

    • Acts as an electrical insulator.

    • Increases speed of conduction of nerve impulses.

    • Protects axons.

Gray and White Matter

  • Gray Matter: Contains nerve cell bodies and dendrites, appears gray due to ribosomes that absorb light.

  • White Matter: Composed of myelinated axons; appears white due to the myelin sheath.

  • Locations:

    • Brain: Gray matter on the outside, white matter deeper.

    • Spinal Cord: White matter on the outside, gray matter (butterfly shape) on the inside.

Axons

  • Myelinated Axons: Faster conduction of nerve impulses.

  • Unmyelinated Axons: Slower conduction.

  • Function of Myelin: Provides a guiding tube for regrowth of damaged axons in the peripheral nervous system; repair rarely occurs in the central nervous system (CNS).

Nerve Impulse Conduction

  • Dendrocytes: Cells responsible for myelinating axons in the CNS.

  • Nerve Damage: Loss of conduction beyond the area of injury. Axons may grow back but won't reattach completely.

Function of the Nervous System

  • Synapses: Junctions between nerve cells or between nerve cells and other cells.

  • Role in Learning: Formation and breaking of synapses relate to memory and skill acquisition.

Electrical Properties of Neurons

  • Definition: Neurons are electrically excitable due to the voltage difference across their membrane.

  • Resting Neuron Voltage: Approximately -70 millivolts; more positive outside, more negative inside due to ion concentration differences.

    • Key Ions: Sodium, potassium, chloride, and organic phosphates.

Types of Signals in Neurons

  • Action Potentials: Long-distance signals traveling along axons.

  • Graded Potentials: Local changes in membrane potential, occurring in dendrites and cell body.

Ion Channels

  • Role: Proteins that allow ions to cross the neuronal membrane.

  • Types of Ion Channels:

    • Voltage-gated Ion Channels: Open or close in response to voltage changes.

    • Ligand-gated Ion Channels: Open or close when a chemical binds to them.

    • Mechanically gated Ion Channels: Respond to mechanical stimuli (e.g., touch).

Resting Membrane Potential

  • Definition: The voltage across the membrane of a resting neuron (-70 mV).

  • Polarization: More positive outside the cell and more negative inside.

Graded Potentials vs. Action Potentials

  • Graded Potentials:

    • Small deviations from resting membrane potential.

    • Can be hyperpolarizing (more negative) or depolarizing (less negative).

  • Action Potentials:

    • Larger, all-or-nothing events that follow a specific sequence (depolarization and repolarization).

Action Potential Generation

  • Process involves:

    • Threshold: When graded potentials reach -55 mV, an action potential will fire.

    • Sequence:

    1. Voltage-gated sodium channels open, sodium enters, depolarizing the cell.

    2. Sodium channels close, potassium channels open, potassium exits, repolarizing the cell.

  • All-or-nothing response: Once initiated, an action potential propagates along the axon.

Propagation of Action Potential

  • Mechanism: The action potential travels along the axon through a domino effect.

  • Comparison:

    • Continuous Conduction: In unmyelinated fibers; propagates along the entire length of the axon.

    • Saltatory Conduction: In myelinated fibers; action potential jumps from node to node (nodes of Ranvier) for faster transmission.

Speed of Nerve Impulses

  • Factors affecting speed: Diameter of axon and presence of myelin sheath.

  • Types of fibers:

    • A Fibers: Largest diameter, myelinated, fast speed (up to 130 m/s); sensory and motor neurons.

    • B Fibers: Smaller diameter, myelinated; visceral information.

    • C Fibers: Smallest diameter, unmyelinated; internal sensory information.

Variety of Sensory Input

  • Detection differences in touch:

    • Frequency of Impulses: More pressure generates higher frequency of action potentials.

    • Number of Neurons Involved: Greater activation with stronger touches.

Synapses

  • Definition: Junctions between neurons or between a neuron and another cell (muscle or gland).

  • Types of Synapses:

    • Chemical Synapse: Predominantly uses neurotransmitters for communication between neurons (one-way flow).

    • Electrical Synapse: Less common; direct flow of ions between cells via gap junctions.

  • Components of a synapse:

    • Presynaptic Neuron: The neuron before the synapse releasing neurotransmitter.

    • Postsynaptic Neuron: The neuron receiving the signal after the synapse.

    • Synaptic Vesicles: Contain neurotransmitters that are released into the synaptic cleft upon stimulation.

Local Anesthetics and Nerve Function

  • Local anesthetics block voltage-gated sodium channels, preventing action potentials from being generated, thus blocking pain signals.

  • Examples: Novocaine, Lidocaine, Benzocaine.

Summary of Key Concepts

  • Importance of the myelin sheath in nerve impulse transmission and nerve healing.

  • Understanding action potentials as all-or-nothing events crucial for nervous system communication.

  • The role of synapses in processing information and the mechanism of neurotransmitter release.